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Rickets: Calcium, Genes, etc - Jan 2013

Indian J Endocrinol Metab. 2013 Jan-Feb; 17(1): 1–4.

doi:  10.4103/2230-8210.107789PMCID: PMC3659873

Rickets: Twists and turns in the Gordian knot

Department of Endocrinology, Command Hospital, Southern Command, Pune, Maharashtra, India
1Department of Endocrinology, Bharti Hospital and B.R.I.D.E., Karnal, Haryana, India
Corresponding Author: Col. M. K. Garg, Department of Endocrinology, Command Hospital, Southern Command, Pune, Maharashtra, India. E-mail: mkgargs@gmail.com

Traditional scientific wisdom teaches that rickets, so plentiful in India, is caused by vitamin D deficiency and cured by vitamin D replacement. Rickets also occurs in the presence of vitamin D sufficiency, as in hypophosphatemic and hypocalcemic rickets; this, however, is not the topic of this editorial. It is still not understood why some children develop rickets, while others do not, even in the face of severe Vitamin D deficiency. This editorial tries to probe the recent twists and turns that researchers have taken while trying to unravel this Gordian knot.



The role of calcium deficiency on the background of vitamin D deficiency as the causation of rickets has been highlighted in a recent article by Aggarwal, et al.1" rel="external nofollow">1 They compared 67 cases of rickets with 68 healthy controls. Among cases, serum 25-hydroxy vitamin D 25(OH) D levels were comparable, but calcium intake, total and from diary intake was significantly lower as compared to controls. They concluded that rickets develops when low dietary calcium intake coexists with a low or borderline vitamin D nutrition status. However, they have not highlighted another interesting finding: Cases of rickets mount robust parathyroid hormone (PTH) response as compared to controls with same vitamin D levels. This was again explained by calcium deficiency which exacerbated PTH response in the presence of vitamin D deficiency. A similar observation has been made by an Indian study among elderly population, in which more than half of population did not mount PTH response in spite of severe vitamin D deficiency 25(OH) D <10 ng/ml.2" rel="external nofollow">2 A recently published study also substantiated this observation by analyzing lab results of more than 300,000 pairs of 25(OH) D and PTH levels in a population. They also found that >50% of patients do not mount PTH response.3" rel="external nofollow">3 Similar observation in two different populations with different calcium intake suggests that there are other mechanisms/factors responsible for PTH response rather than calcium intake. It is also likely same mechanisms/factors may also contribute to development of rickets in patients with vitamin D deficiency rather than dietary calcium deficiency.


Endogenous vitamin D is synthesized in epidermis. Exogenously, vitamin D can be derived from vegetable and animal sources. Vitamin D is produced in the skin by a UVB-mediated, photolytic, non-enzymatic reaction that converts 7-dehydrocholesterol to pre-vitamin D3. Pre-vitamin D3 undergoes a subsequent non-enzymatic, thermal isomerization conversion to vitamin D3 in the skin. Vitamin D3 enters circulation and gets bound to vitamin D binding protein and is taken up by liver. In the hepatic parenchyma, vitamin D3 is converted by one of several cytochrome P450 (CYP2R1, CYP2D11, CYP2D25) to 25(OH) D. 25(OH) D is the most plentiful and stable metabolite of vitamin D in human serum. This 25(OH) D is filtered in renal glomerulus and internalized in proximal convoluted tubules with the help of megalin and converted to 1,25 dihydroxy vitamin D1,25(OH)2D by mitochondrial 1-αhydroxylase (CYP27B1). This reaction requires NADPH and ferrodoxin for electron transfer during hydroxylation reaction. 1,25(OH)2D serves as a high affinity ligand for the vitamin D receptor (VDR) in target tissues where it acts to modulate expression of vitamin D-directed genes. VDR is present in most tissues and cells in body including parathyroid cells. 1,25(OH)2D is metabolized by 24-hydroxylase (CYP24A1) enzyme to relatively inactive metabolites like 24,25 dihydroxy vitamin D and calcitroic acid.4" rel="external nofollow">4 Calcium sensing receptors are also present on parathyroid cells, which modulate PTH secretion by binding with divalent cations.


Any mechanism which affects the responsiveness of parathyroid cells will modify PTH secretory response. Activated vitamin D modifies PTH secretory response. Obviously, the same mechanism will also be operative in other VDR responsive cells. Hence, we speculate that patients who mount PTH response will have a less effective protective mechanism against vitamin D deficiency and will negatively affect calcium absorption from gut. On the contrary, those with a protective mechanism will suppress the PTH response and have more efficient absorption of dietary calcium. This suggests that it is the calcium absorptive mechanism, rather than dietary calcium intake, which predisposes an individual with vitamin D deficiency to develop rickets and PTH response.


What could be the underlying mechanism(s) of this absent PTH response to vitamin D deficiency? Answer to this can explain above enigma and it may lie in genetic polymorphism. There are several genes involved in vitamin D metabolism namely GC (group component), DHCR7 (7-dehydrochlesterol reductase), NADSYN1 (nicotinamide adenine dinucleotide synthetase), CYP2R1, CYP27B1, CYP24A1, and VDR gene. 25(OH) D has high heritability (28-80%)5" rel="external nofollow">5 but others disagree with this.6" rel="external nofollow">6


Single-nucleotide polymorphisms (SNPs) in the gene GC (rs2282679, rs4588, and rs2282679) and a non-synonymous SNP (rs7041 and rs1155563), SNP in NADSYN1/DHCR7 (rs3829251 and rs1790349), and SNP in CYP2R1 (rs2060793) have been associated with lower 25(OH) D concentrations in European and Chinese ancestry.7,8" rel="external nofollow">7,8 Similar findings have been reported in Canadian adults of east Asian, but not south Asian, origin.9" rel="external nofollow">9 A systemic review of various genomic studies suggested that the optimal concentrations of 25(OH) D may vary according to genotype.10" rel="external nofollow">10 SNPs involved in above genetic loci may affect 25(OH) D levels but do not affect 1,25(OH)2D11" rel="external nofollow">11 and are unlikely to affect its action. Moreover, animal models with knock out loci did not develop rickets and PTH response even in the presence of low 25(OH) D concentration.4" rel="external nofollow">4 Hence, these SNPs will not be able to untangle the Gordian knot of rickets pathophysiology.


Some polymorphisms of CYP27B1 (1-αhydroxylase enzyme) may affect the efficiency of process of generating active 1,25(OH)2D. CYP27B1-1260 promoter polymorphism (rs10877012) has a substantial impact on 1,25-(OH) 2 D serum levels, with higher levels noted with AA variant than CC variant.12" rel="external nofollow">12 Other studies have also reported low levels of 1,25(OH)2D with the same SNP.10" rel="external nofollow">10 More evidence comes from a study of prostatic carcinoma, where one CYP27B1 tag SNP (rs3782130) and two CYP24A1 tag SNPs (rs927650 and rs2762939) were associated with adverse outcome,11" rel="external nofollow">11 indicating low efficiency of 1,25(OH)2D on tissue effect.

Another enzyme, CYP24A1, also plays an important role in metabolism of 25(OH) D and 1,25(OH)2D. A SNP of CYP24A1 (rs6013897) was associated with low level of 25(OH) D13" rel="external nofollow">13 indicating that increased activity of this enzyme can decrease active vitamin D. A study among South Asians showed increased activity of 24 hydroxylase enzyme in cultured in skin fibroblast and its association with lower serum 25OHD.14" rel="external nofollow">14 It can be hypothesized that in subjects with less efficient CYP27B1 and relatively efficient CYP24A1 system, there will less generation of active vitamin D in the face of vitamin D deficiency or SNPs described in THE above paragraph. Decreased level of active vitamin D will have decreased physiological effects. A negative 1,25(OH)2D response element is present on the promoter region of the PTH gene. Hence, decreased levels of active vitamin D will upregulate PTH gene expression and increase PTH level, simultaneously decreasing calcium absorption and explaining the rachitic Gordian Knot.


Can an internal factor other than those mentioned above be involved in the vitamin D-PTH-calcium axis? Nuclear factor kappa-B has been implicated in downregulation of CYP27B1 gene expression.15" rel="external nofollow">15 Indian subjects have higher levels of inflammatory markers compared to Caucasians and Europeans.16" rel="external nofollow">16 Hence, it can be speculated that those with underlying inflammatory conditions will be more predisposed to rickets with similar 25(OH) D levels. Study of inflammatory markers in the subjects with and without rickets may reveal this pathogenetic mechanism.


VDR gene polymorphism has been reported to be a determinant of bone formation and intestinal calcium absorption. Most frequently studied are three adjacent restriction fragment length polymorphisms (RFLPs) for BsmI, ApaI, and TaqI at the 3′ end of the VDR gene and one RFLP in the start codon of VDR gene—FokI. A meta-analysis revealed that the most common haplotype for the VDR gene, regardless of ethnicity, is baT, followed by BAt and bAT in Caucasians and bAT and BaT in Asians.17" rel="external nofollow">17 An Indian study reported Ft, TT, and Aa as the most common polymorphism of VDR, and FTA followed by fta as common haplotype.18" rel="external nofollow">18 Many studies have assessed variable number of RFLPs and its effect on bone mineral density, calcium absorption, and PTH levels. Studies from India and Turkey reported no association of any of VDR polymorphism (BsmI and FokI from India; and ApaI and TaqI from Turkey) in patients with osteomalacia or rickets,19,20" rel="external nofollow">19,20 while a study from China found that FF genotype was significantly associated with vitamin D deficiency rickets.21" rel="external nofollow">21 Lorentzon, et al. reported that ApaI genotype Aa was found to be related to higher level of PTH in healthy Caucasian girls.22" rel="external nofollow">22 Another study revealed a significant association of VDR FokI with PTH levels.23" rel="external nofollow">23 Marco, et al. reported that BB genotype (BsmI) had less severe secondary hyperparathyroidism in predialysis patients.24" rel="external nofollow">24

However, these polymorphisms occur in the nonfunctional region of VDR and may indicate linkage disequilibrium with other truly functional polymorphisms elsewhere in the VDR gene.25" rel="external nofollow">25 Recently, a study reported functional haplotype alleles of the 5′ 1a/1e, 1b promoter region and of the 3′ untranslated region was associated with 15% lower mRNA level of VDR expression and this could impact the vitamin D signaling efficiency.26" rel="external nofollow">26 Three VDR tag SNPs (rs3782905, rs7299460, and rs11168314) and one SNP in VDR (rs10735810)11" rel="external nofollow">11 were associated with adverse outcome in patients with prostatic carcinoma indicating low efficiency of these receptor polymorphism.14" rel="external nofollow">14 These studies indicate that VDR polymorphisms play an important role in bone and parathyroid gland behavior, leading to differential response due to a likely tissue-specific effect of the VDR response to calcitriol.27" rel="external nofollow">27 Hence, in subjects with VDR polymorphisms which are less efficient, this will adversely impact osteoblastic cells and parathyroid cells, and may untie the Gordian knot.


Finally, polymorphism of calcium sensing receptor which is activated at higher calcium levels will have exaggerated PTH response at lower level of calcium. The 986S polymorphism of the CASR has recently been associated with higher serum ionized calcium levels.28" rel="external nofollow">28 but three other CaSR coding region polymorphisms (Ala986Ser, Arg990Gly, and Gln1011Glu) have no major influence on indices of calcium homeostasis in this female population.29" rel="external nofollow">29 More studies are required in this area evaluating not only interaction of calcium and CaSR but other nutritional divalent cations like magnesium, which may be contributing to the Gordian knot.


Rickets/osteomalacia can be termed a life style disease. All humans had ample exposure to sunlight in aboriginal days. With evolution and modernization, the population started wearing clothes, moved into indoor dwellings, air conditioned offices, and cars. Hence, there started a trend of less and less sun exposure and synthesis of vitamin D. Nature is a great healer. Possibly there occurred genetic changes to overcome this vitamin D deficiency by improving efficiency of the vitamin D–PTH–calcium system. This is reflected in genetic polymorphism. However, individuals without these protective mechanisms are predisposed to clinical disease, and will develop rickets and osteomalacia when challenged by Vitamin D deficiency. More research is needed, however, before we have clear answers to the question raised at the beginning of this editorial. And this will open the Gordian knot of differential response to 25(OH) D levels in given individuals.


1. Aggarwal V, Seth A, Aneja S, Sharma B, Sonkar P, Singh S, et al. Role of calcium deficiency in development of nutritional rickets in Indian children: A case control study. J Clin Endocrinol Metab. 2012;97:3461–6. PubMed: 22893720
2. Marwaha RK, Tandon N, Garg MK, Kanwar R, Narang A, Sastry A, et al. Vitamin D status in healthy Indians aged 50 years and above. J Assoc Physicians India. 2011;59:706–9. PubMed: 22616336
3. Valcour A, Blocki F, Hawkins DM, Rao SD. Effects of age and serum 25-OH-Vitamin D on serum parathyroid hormone levels. J Clin Endocrinol Metab. 2012 In Press.
4. Christakos S, Ajibade DV, Dhawan P, Fechner AJ, Mady LJ. Vitamin D: Metabolism. Rheum Dis Clin North Am. 2012;38:1–11. PubMed: 22525839
5. Bu FX, Armas L, Lappe J, Zhou Y, Gao G, Wang HW, et al. Comprehensive association analysis of nine candidate genes with serum 25-hydroxy vitamin D levels among healthy Caucasian subjects. Hum Genet. 2010;128:549–56. PubMed: 20809279
6. Berry D, Hyppönen E. Determinants of vitamin D status: Focus ongenetic variations. Curr Opin Nephrol Hypertens. 2011;20:331–6. PubMed: 21654390
7. Ahn J, Yu K, Stolzenberg-Solomon R, Simon KC, McCullough ML, Gallicchio L, et al. Genome-wide association study of circulating vitamin D levels. Hum Mol Genet. 2010;19:2739–45. PMCID: PMC2883344 PubMed: 20418485
8. Lu L, Sheng H, Li H, Gan W, Liu C, Zhu J, et al. Associations between common variants in GC and DHCR7/NADSYN1 and vitamin D concentration in Chinese Hans. Hum Genet. 2012;131:505–12. PubMed: 21972121
9. Gozdzik A, Zhu J, Wong BY, Fu L, Cole DE, Parra EJ. Association of vitamin D binding protein (VDBP) polymorphisms and serum 25(OH) D concentrations in a sample of young Canadian adults of different ancestry. J Steroid Biochem Mol Biol. 2011;127:405–12. PubMed: 21684333
10. McGrath JJ, Saha S, Burne TH, Eyles DW. A systematic review of the association between common single nucleotide polymorphisms and 25-hydroxyvitamin D concentrations. J Steroid Biochem Mol Biol. 2010;121:471–7. PubMed: 20363324
11. Ahn J, Albanes D, Berndt SI, Peters U, Chatterjee N, Freedman ND. Prostate, Lung, Colorectal and Ovarian Trial Project Team. Vitamin D-related genes, serum vitamin D concentrations and prostate cancer risk. Carcinogenesis. 2009;30:769–76. PMCID: PMC2675652 PubMed: 19255064
12. Lange CM, Bojunga J, Ramos-Lopez E, von Wagner M, Hassler A, Vermehren J, et al. Vitamin D deficiency and a CYP27B1-1260 promoter polymorphism are associated with chronic hepatitis C and poor response to interferon-alfa based therapy. J Hepatol. 2011;54:887–93. PubMed: 21145801
13. Wang TJ, Zhang F, Richards JB, Kestenbaum B, van Meurs JB, Berry D, et al. Common genetic determinants of vitamin D insufficiency: A genome-wide association study. Lancet. 2010;376:180–8. PMCID: PMC3086761 PubMed: 20541252
14. Awumey EM, Mitra DA, Hollis BW, Kumar R, Bell NH. Vitamin D metabolism is altered in Asian Indians in the southern United States: A clinical research center study. J Clin Endocrinol Metab. 1998;83:169–73. PubMed: 9435436
15. Ebert R, Jovanovic M, Ulmer M, Schneider D, Meissner-Weigl J, Adamski J, et al. Down-regulation by nuclear factor kappaB of human 25-hydroxyvitamin D3 1alpha-hydroxylasepromoter. Mol Endocrinol. 2004;18:2440–50. PubMed: 15243130
16. Yajnik CS, Joglekar CV, Lubree HG, Rege SS, Naik SS, Bhat DS, et al. Adiposity, inflammation and hyperglycaemia in rural and urban Indian men: Coronary risk of insulin sensitivity in Indian subjects (CRISIS) study. Diabetologia. 2008;51:39–46. PubMed: 17972060
17. Thakkinstian A, D’Este C, Attia J. Haplotype analysis of VDR gene polymorphisms: A meta-analysis. Osteoporos Int. 2004;15:729–34. PubMed: 15057510
18. Bid HK, Mishra DK, Mittal RD. Vitamin-Dreceptor (VDR) gene (Fok-I, Taq-I and Apa-I) polymorphisms in healthy individuals from north Indian population. Asian Pac J Cancer Prev. 2005;6:147–52. PubMed: 16101324
19. Ray D, Goswami R, Gupta N, Tomar N, Singh N, Sreenivas V. Predisposition to vitamin D deficiency osteomalacia and rickets in females is linked to their 25(OH) D and calcium intake rather than vitamin D receptor gene polymorphism. Clin Endocrinol (Oxf) 2009;71:334–40. PubMed: 19094076
20. Ak DG, Kahraman H, Dursun E, Duman BS, Erensoy N, Alagöl F, et al. Polymorphisms at the ligand binding site of the vitamin D receptor gene and osteomalacia. Dis Markers. 2005;21:191–7. PubMed: 16403954
21. Lu HJ, Li HL, Hao P, Li JM, Zhou LF. Association of the vitamin D receptor gene start codon polymorphism with vitamin D deficiency rickets. Zhonghua Er Ke Za Zhi. 2003;41:493–6. PubMed: 14746673
22. Lorentzon M, Lorentzon R, Nordström P. Vitamin D receptor gene polymorphism is related to bone density, circulating osteocalcin, and parathyroid hormone in healthy adolescent girls. J Bone Miner Metab. 2001;19:302–7. PubMed: 11498732
23. Laaksonen MM, Outila TA, Karkkainen MU, Kemi VE, Rita HJ, Perola M, et al. Associations of vitamin D receptor, calcium-sensing receptor and parathyroid hormone gene polymorphisms with calcium homeostasis and peripheral bone density in adult Finns. J Nutrigenet Nutrigenomics. 2009;2:55–63. PubMed: 19690432
24. Marco MP, Martínez I, Amoedo ML, Borràs M, Saracho R, Almirall J, et al. Vitamin D receptor genotype influences parathyroid hormone and calcitriol levels in predialysis patients. Kidney Int. 1999;56:1349–53. PubMed: 10504487
25. Uitterlinden AG, Fang Y, van Meurs JB, van Leeuwen H, Pols HA. Vitamin D receptor gene polymorphisms in relation to Vitamin D related disease states. J Steroid Biochem Mol Biol. 2004;89-90:187–93. PubMed: 15225770
26. Fang Y, van Meurs JB, D'Alesio A, Jhamai M, Zhao H, Rivadeneira F, et al. Promoter and 3′-untranslated-region haplotypes in the vitamin D receptor gene predispose to osteoporotic fracture: The rotterdam study. Am J Hum Genet. 2005;77:807–23. PMCID: PMC1271389 PubMed: 16252240
27. Alvarez-Hernandez D, Naves-Diaz M, Gomez-Alonso C, Coto E, Cannata-Andia JB. Tissue-specific effect of VDR gene polymorphisms on the response to calcitriol. J Nephrol. 2008;21:843–9. PubMed: 19034868
28. Lorentzon M, Lorentzon R, Lerner UH, Nordström P. Calcium sensing receptor gene polymorphism, circulating calcium concentrations and bone mineral density in healthy adolescent girls. Eur J Endocrinol. 2001;144:257–61. PubMed: 11248745
29. Harding B, Curley AJ, Hannan FM, Christie PT, Bowl MR, Turner JJ, et al. Functional characterization of calcium sensing receptor polymorphisms and absence of association with indices of calcium homeostasis and bone mineral density. Clin Endocrinol (Oxf) 2006;65:598–605. PubMed: 17054460
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